Digital Subscriber Line (DSL) systems are high-bandwidth technologies that use the existing copper-cable telephone lines. Asymmetric Digital Subscriber Line (ADSL) is particularly attractive for consumer Internet applications where most of the data traffic is downloaded to the customer. Upstream bandwidth for uploading data can be reduced to increase downstream bandwidth since most Internet traffic is downstream traffic.
Examples of multicarrier transmission techniques used in DSL systems include Quadrature Amplitude Modulation (QAM) and a version of QAM known as Discrete Multitone (DMT). In DMT, a channel comprises sub-channels, also referred to as frequency bins, bins, or sub-carriers. Each sub-channel has sine and cosine frequencies that are integer multiples of a common frequency, the inverse of this common frequency being the symbol period. The amplitude and phase of each bin or sub-carrier represents a group of information bits. Each sub-channel is encoded to a point in a constellation having points wherein each point is unique for each combination of bits. For example, a sub-channel carrying a 2-bit symbol would be encoded using a 4-point constellation, and a 3 bit symbol in another sub-channel would be encoded for an 8-point constellation.
Recently, the ITU-T standards body defined a new generation of ADSL, also referred to as G.992.3 or ADSL2. The previous generation of ADSL standards (G.992.1) included an Annex C, which describes techniques for operating ADSL in the same cable binder as TCM-ISDN signaling. A similar annex is being defined for G.992.3.
G.992.3 provides three mandatory link states or power management states or modes. One is referred to as L0 state and is named “Full On.” The ADSL link is fully functional in L0 meaning normal data transmission of payload data can occur. Another state is the L2 state and is named “Low Power.” In this state, the ADLS link is active, but a low power signal conveying background data is sent from the ATU-C (transmitter) to the ATU-R (receiver). A normal data carrying signal is transmitted from the ATU-R to the ATU-C. A third state is the L3 state called “Idle” in which there is no signal transmitted so that the transceiver ATU may be powered or unpowered. For each of the L0 and L2 states, there is a bitmap identifying the sub-carriers used in that mode, and associated bitmap tables including a bit allocation table and a gain table for each state are stored in memory. The bits per bin and gain factor per bin for the L0 mode are determined during initialization in order to achieve a signal-to-noise ratio (SNR) margin. The SNR margin is the maximum increase (in dB) of the received noise power, such that the ATU can still meet all the target bit error rates (BERs) over all the frame bearers. The bit allocation table is the number of bits, bi, allocated for each frequency or subcarrier i in ascending order from one to the number of sub-carriers used −1. For each sub-carrier, there is also a relative gain defined, gi, stored in a gain table, indexed in the same order as the bit allocation table. The relative gain may also be referred to as a gain scale factor, a transmit gain factor, a fine gain adjustment, or a fine tune gain. The gi values define a scaling of the root mean square (rms) subcarrier power levels relative to a downstream reference transmit power spectral density (REFPSDs) level. For G.992.3, gi values in dB=20 log (gi in linear scale). Additionally, a tone ordering table for each state may also be used, and it defines the sequence in which subcarriers are assigned bits from an input bit stream.
Upon transition of the system to L2 mode, other bit and gain tables (bi, gi) are provided by the receiver in order to achieve a lower data rate for the reduced power level. For example, in G.992.3, the transmitter sends an L2 Request message for entry into the L2 state including a minimum power cutback value (minimum PCBds value in dB) and a maximum power cutback value (maximum PCBds value in dB). The receiver will send either a L2 Reject or a L2 Grant message or command. An L2 Reject command may be sent, for instance, if the cutback values are not within an allowed range or because the current line and noise conditions cannot satisfy the desired operating condition. An L2 Grant message includes the actual power cutback value and the bitmap with the bits and gain tables to be used by the ATUs in the downstream direction. The bit and gain tables for L0 mode (normal data transmission) need to be stored so that the modem can quickly resume L0 mode data transmission without retraining. This imposes memory storage requirements for two (2) bitmaps in a modem to accommodate both the L0 and L2 modes.
Additionally, there are certain system environments that use a plurality of bitmaps during normal data transmission. Annex C addresses the operation of ADSL transceivers operating over copper telephone lines in the same cable binder as Time Compression Multiplexing (TCM)-Integrated Services Digital Network (ISDN) signaling. These Annex C transceivers use 2 bitmaps during normal transmission, to deal with near-end (NEXT) and far-end (FEXT) crosstalk. For the transmitter, the FEXTC bitmap is used in 128 DMT symbols in a hyperframe of 345 DMT symbols and the NEXTC bitmap is used in 217 DMT symbols in a hyperframe of 345 DMT symbols. For the L2 mode in Annex C, memory space for the bitmap tables of 2 bitmaps must be accommodated instead of just for one. L2 mode imposes higher demand for memory space, especially when the number of sub-carriers (NSC) is increased.
It is therefore desirable to reduce the memory requirement in L2 mode operation.